1. Field of the Invention
This invention relates to a motor oil composition for internal combustion engines. More particularly, this invention relates to an SAE 5W-X multi-grade motor oil where the 5W base oil is blended to produce, for example, oils of SAE number 20, 30, 40 or 50. In one aspect this invention relates to a SAE 5W-40 grade oil which has a low phosphorus and nitrogen content, low consumption rate, minimal high temperature volatility, and which demonstrates retained viscosity characteristics over extended use.
2. Discussion of the Art
Lubricating oils must provide minimal frictional wear in an engine over a wide range of operating temperatures. These engine temperatures can range from below freezing during cold weather starting to above 400.degree. F. (200.degree. C.) during severe usage. A multi-grade blend of oils and additives is usually required to provide both low temperature cranking properties and thermal stability at higher temperatures.
A continuing problem in the art is formulating a multi-grade oil which will maintain all of the desired characteristics, i.e. low viscosity at starting temperatures and high viscosity with low volatility at operating temperatures, without sacrificing or compromising any of them. Although such oil blends could be formulated from natural petroleum stocks by the addition of viscosity index improvers, a large amount of lower molecular weight polymers would be needed to provide the lower viscosity at low operating temperatures. Unfortunately the shear stability of these lower molecular weight polymers is poor and the polymer chains often cleave at higher temperatures, destroying the 5W properties of the oil.
Another problem associated with formulations containing a 5W oil is consumption and short useful life. The oil blends necessary for low viscosity at low temperatures usually suffered from high volatility at higher engine operating temperatures.
The viscous detergent-dispersant additives also had to be kept to a minimum amount to meet the viscosity requirements of a 5W oil. Such minimal amounts were often insufficient to keep the engine clean over extended use.
Many petroleum additives contain phosphorus compounds, yet phosphorus is known to poison catalysts used in automobile exhaust converters. Increasing restrictions on automobile emissions will require that modern multi-grade motor oils contain little or no phosphorus while maintaining desirable viscosity characteristics. An increasing demand for light viscosity base stocks will also be seen as the auto industry moves toward cars with small four-cylinder engines. Improved fuel economy is a benefit of using the lower viscosity oil, as well as easier cold starting, sustained firing and extended battery life.